The present invention concerns a photodetection device as well as a procedure for the manufacture of this device.
As will be seen better as follows, the device that is the objective of the present invention offers a wide range of selection for the wave length, extreme speed and great sensitivity.
It is applicable to any field liable to take advantage of at least one of these qualities such as, for example, the microscopic detection of molecules and, more specifically, very high speed optical telecommunications, greater than or equal to 100 Gbits per second.
MSM (Metal-Semiconductor-Metal) type photodetectors are generally quite simple to manufacture, they are easily fitted into field effect transistors and allow relatively high speed to be obtained but to the detriment of performance. Hereinafter, some MSM photodetectors are considered that are known as well as their drawbacks.
In a known photodetector based on InGaAs, whose distance between the electrodes is 1 xcexcm, the transit time of the holes is around 10 ps, which corresponds to a cut-off frequency of less than 20 GHz. Therefore the distance between the electrodes must be reduced in order to cut down the transit time for the holes. When the distance between the electrodes drops below 0.1 xcexcm, the transport can no longer be considered as stationary. The transit time then becomes much lower than 1 ps.
The masking of the active zone by the electrodes is one of the main drawbacks of the known MSM structures and limits their quantum yield. Furthermore, because of the limited absorption of the materials used in these structures (the length of absorption is greater than 1 xcexcm), the thickness of the absorption zone must be limited so as to prevent the creation of charge carriers far away from the electrodes. The quantum efficiency of the known photodetectors, since they have a range between the electrodes of less than 0.1 xcexcm, is therefore extremely bad.
On the contrary, the known MSM structures, whose external quantum yield is good, have a low speed.
But nowadays a super-fast photodetector (whose response time is less than 1 ps) is a crucial element for very high speed optical telecommunications (100 Gbits/s and above). The performance levels sought include great sensitivity and broadband, at wavelengths between 1.3 xcexcm and 1.55 xcexcm. Whatever type of photodetector it may be (P(I)N diode or Metal-Semiconductor-Metal structure), the target of high speed forces the distance between the electrodes to be short (less than 100 nm) and that the light to be detected must be absorbed in a minimum volume.
Hence, the bulk InGaAs semiconductor has a characteristic absorption length of around 3 xcexcm at a wavelength of 1.55 xcexcm.
In the PIN diodes and in the MSM structures, the reduction of the transit time for the charge carriers is directly linked to a drop in the external quantum yield.
The design of the known photodetectors therefore is necessarily the subject of a compromise between yield and speed.
The device that is the subject of the invention aims to radically question this compromise and uses a vertical microresonator, which allows, for example, a quantum yield of over 70% to be attained in a low capacity structure, whose range between the electrodes may be less than 50 nm and may lead to a bandwidth of over 1 THz.
The principle for a device in accordance with the invention consists of concentrating the light that we may wish to detect in a resonant manner, in a low volume MSM type structure, by using the fast drop in evanescent modes excited in the Metal/Semiconductor interface.
The surface plasmon modes allow this aim to be achieved.
Unlike the known structures, the plasmons do not spread horizontally (that is to say in parallel to the substratum of the structure), but rather they remain confined along the vertical surface of the electrodes in the structure.
In a precise manner, the aim of the present invention is a photodetection device intended to detect an incident light with a predefined wavelength, propagating in a propagation medium, with this device being characterised by the fact that it includes an electrically insulating layer that does not absorb this light and, on this layer, at least one element, including a semiconductor material, and at least two biasing electrodes, intended to be carried respectively to potentials that are different from one another, with the electrodes surrounding the element, with the set formed by the element and the electrodes being adapted to absorb the incident light (in other words, the element and/or the electrodes are suitable for absorbing this light), with the element and the electrodes having a shape that is substantially parallelepipedal and extending following the same direction, with the dimmensions of the electrodes and the element, counted transversally to this direction, being chosen according to the predefined wavelength, in such a way as to increase the light intensity in the set formed by the element and the electrodes with respect to the incident light, by making at least one of two modes resonate, that is to say a first mode which is a surface plasmon mode and which made to resonate between the interfaces that this set includes with the insulating layer and the propagation medium, with the resonance of this first mode taking place at the interface between the element and at least one of the electrodes, with this first mode being excited by the component of the magnetic field associated with the incident light, a component that is parallel to the electrodes, and a second mode which is a transverse electrical mode of an optical waveguide which is perpendicular to the insulating layer and includes the two electrodes, with this second mode being excited by the component of the electric field associated with the incident light, a component which is parallel to the electrodes.
Preferentially, when the surface plasmon mode is made to resonate, the width of each element, counted perpendicularly to the direction of the electrodes, is less than xcex and greater than 0.02xc3x97xcex, where xcex is the wavelength of the incident light and the thickness of each element is less than xcex/(2n), where n is the average refractive index for each element.
According to a first particular form for building the device that is the subject of the invention, the electrodes are made of the same electrically conductive material and are the same height, counted perpendicularly to the insulating layer.
According to a second particular form for building it, the electrodes have at least one of the following two properties (a) they are made of different electrically conductive materials and (b) they have different heights, counted perpendicularly to the insulating layer, in such a way that the resonance takes place essentially on the side of the electrode which collects the slow charge carriers at the time of the biasing of the electrodes.
The element that the device carries may include a semiconductor heterostructure.
According to a particular mode for its construction, the device that is the subject of the invention includes several elements and electrodes that alternate on the insulating layer, with each electrode being made of a single metal or of two different metals.
In this case, in a first particular mode for its implementation, the electrodes are intended to be carried to potentials which grow from one end electrode to the other end electrode in the set of electrodes.
The device that is the subject of the invention may then also include a resistive material, for stabilising potentials, which is in contact with the electrodes and runs from one end electrode to the other end electrode in the set of electrodes. The latter allows the set of elements to be polarised under a high voltage.
In a second particular mode for its implementation, the electrodes are intended to be carried to potentials whose absolute values are equal and whose signs alternate.
According to a preferred mode for the implementation of the device that is the subject of the invention, this device also includes a means for reflection planned to reflect the light that is not absorbed, crossing the insulating layer, with the thickness of this insulating layer being chosen so that the light reflected by the means for reflection will be in phase with the light waves present in the set formed by each element and the electrodes and will participate in the resonance.
In a first example, the device that is the subject of the invention is intended to detect an incident light whose wavelength is approximately 0.8 xcexcm, this device is formed on a substratum of GaAs, the element is made of GaAs, the electrodes are made of Ag, the insulating layer is made of AlAs, or out of an AlxGa1-xAs material, with x being chosen in such a way that this material will not absorb the incident light but will allow a selective etching of the GaAs, and the reflection means is a multiple layer AlAs/AlGaAs mirror.
In a second example, the device is intended to detect an incident light whose wavelength is approximately 1.55 xcexcm, this device is formed on a substratum made of InP, the element is made of InGaAs, the electrodes are made of Ag, the insulating layer is made of AlInAs and the means for reflection is a multiple layer mirror made of GaInAsP/InP or AlGaInAs/AlInAs. As a variation, the device is formed on a GaAs substrate, with the element being made of an InGaAsNSb alloy, the electrodes are made of Ag, the insulating layer is made of AlAs, or in an AlxGa1-xAs material, with x being chosen in such a way that this material will not absorb the incident light but will allow a selective etching of the GaAs, and the reflection means is a multiple layer GaAs/AlAs mirror.
In a third example, the device is intended to detect an incident light whose wavelength belongs to the infrared range, and the electrodes are mainly made of Ag or Au in order to absorb the incident light, with the element not absorbing this incident light.
According to a first particular mode for implementing the invention, the propagation medium is air.
According to a second particular mode for implementation, the propagation medium is a light guide parallel to the direction in which the electrodes from each element spread out.
The present invention also concerns a procedure for manufacturing the device that is the subject of the invention, in which a given thickness of the semiconductor material for the element is made to grow on the insulating material, this semiconductor material is etched selectively in order to remove from it portions in the sites corresponding to the electrodes and these electrodes are formed on these sites.
According to a first particular mode for implementing the procedure that is the subject of the invention, the same mask is used to selectively etch the element and then to form the electrodes.
According to a second particular mode for implementing the procedure that is the subject of the invention, a mask is used to selectively etch the element, this mask is removed, the electrodes are formed using at least one metal and the excess material from this metal is removed by means of mechanical or mechanical-chemical polishing.
In this case, according to a particular mode for implementing the invention, the excess metal is removed by means of a selective mechanical or mechanical-chemical polishing of the metal with respect to the element, with this element being made up by a material whose hardness is great compared to that of the metal.
According to a particular mode for implementation, the element includes an upper layer and the excess metal is removed by means of a selective mechanical or mechanical-chemical polishing of the metal with respect to the element, with the upper layer of the element being made up by a material whose hardness is great compared to that of the metal.
Two metals may also be used in order to form the electrodes and deposited successively in an oblique manner with respect to the insulating layer.
It should be noted that, in the present invention, the use of a large number of elements amongst which some electrodes are deposited, instead of the use of a single element placed between two electrodes, allows a network to be built whose electromagnetic modelling is very much simpler.
Then it may be shown that the first mode is made to resonate, which corresponds to some vertical plasmons that are weakly coupled two by two.
The lower and upper ends of the vertical sides of the electrodes have a mirror effect on the plasmons in the metal-semiconductor interface, which allows a Fabry-Pxc3xa9rot type of resonance to be established and so absorb the largest part of the TM (transverse magnetic) polarised incident wave.
The modelling has also allowed the same Fabry-Pxc3xa9rot type resonance phenomenon to be demonstrated for the TE (tranverse electrical) modes of the flat wave guide formed between two electrodes separated by an element and so absorb the largest part of the TE polarised incident light for some suitably chosen parameters of the device.
In the case of TM polarisation and in the case of TE polarisation, total absorption may be obtained by using a Bragg mirror under the electrodes to reflect the wave transmitted in the insulating layer.
The figures for the drawings attached herewith show that, unlike the excitation of horizontal surface plasmons (that is to say those parallel to the insulating layer), the resonance is barely sensitive to the inclination of the incident light wave that we want to detect. It is therefore possible to focus the light wave strongly onto the device and to only activate a small number of elements (for example, 3 to 5), whilst reducing the quantum yield of the device by very little (with respect to the case in which a large number of elements are used).
Furthermore, in the case of TM polarisation, it is recommended that the thickness of each element should be produced accurately whilst that is not the case for the transversal dimensions of each element and of the electrodes.